A. Field
The present invention concerns a method for controlling a load with a predominantly inductive character, for example an electric motor, a generator or the like, whereby in order to feed a phase, of the above-mentioned load, use is made of at least two switched output voltages, applied by one or several power electronic inverters with a given switching, modulation or sampling period, such as an inverter with a direct-current interstage circuit, a buck or boost inverter or the like.
B. Related Art
In general, two basic types of inverters with a direct-current interstage circuit are known, i.e. a Voltage Source Inverter (VSI) employing a DC-link capacitor and providing a switched output voltage, and a Current Source Inverter (CSI), employing a DC-link inductance and providing a switched current waveform at its outputs. Further in the description, only voltage source inverters will be considered.
In a typical voltage source inverter, every inverter leg can be simplified into a 2-way switch applying a switched voltage waveform to a load, as either the positive or the negative DC bus voltage is applied for a short time to one of the phases of said load.
If the negative bus voltage is applied to a certain phase, the current of this phase will decrease, and vice versa, the phase current will usually increase if a positive voltage is applied.
By making use of modulation techniques such as pulse width modulation (PWM) or of known techniques such as hysteresis control, it is possible to provide an average voltage which, within every PWM period, is equal to a reference voltage. Since PWM frequencies are much higher than the frequency of said reference voltage, the reference voltage is reflected by the fundamental wave of the switched pulse pattern. Apart from this fundamental wave, the voltage spectrum at for example the terminals of a motor comprises many higher harmonics. In electrical drive application, this varying output generates additional current harmonics which increases the torque ripple yielding speed variations increase, as well as the losses, which results in motor heating.
A known method for reducing the current ripple resulting from the switched voltage waveform consists in making the switching frequency of the inverter rise. However, the switching frequency is limited to a maximum value which depends both on the power electronics and on the control unit. Moreover, also the losses of the inverter increase as a result of an increasing switching frequency.
Another well-known measure for reducing the current ripple is to provide a choke at the inverter output, or a sinus filter which is formed of the combination of a choke and a capacitor.
Disadvantages of applying a choke at the inverter output are the high cost price, additional losses and the voltage drop. Especially with high-frequency applications such as high-speed drives, the latter disadvantage is important, since the voltage drop depends on the frequency of the fundamental wave.
Parallel converter supply is already known, which is applied for restricting the power rating of every converter in parallel and whereby the legs of the respective inverters are connected directly parallel to one another.
A disadvantage of such a system is that the output voltages of the parallel inverter legs must be identical, since any difference of the output voltage, such as a shift of the PWM-pulses, may cause a dangerously high circulation current or short-circuit within the parallel connection.
In other known systems, every parallel inverter is provided with a separate choke, such that the voltage pulses of each of these respective parallel output legs of the inverters can be shifted.
A known PWM strategy for the parallel inverter output leg consists in applying the same voltage waveform, but whereby the voltage pulses are shifted over half a PWM period.
The major disadvantages of such a strategy consist in that every choke is loaded with a magnetic field which is generated by the full output current of the inverter and in that the switching frequency cannot be reduced.
Other disadvantages of such known methods consist in that the harmonic content is only slightly reduced and in that there is a high voltage drop at high fundamental frequencies.
Another disadvantage of such known methods consists in that the above-mentioned chokes, at high powers, are very sizeable and expensive.
In a previous patent application of the same applicant has already been described a device which remedies some of the aforesaid disadvantages, which device is designed as a differential-mode device which makes sure that the major part of the flux which is generated by the current in one of the inverter legs is counteracted by a flux which is generated by the current in one or several other inverter legs.
Compared to a standard choke with an identical permissible power, the resulting magnetic flux is very low, resulting in low-cost differential-mode devices with small dimensions.